1. Field of the Invention
This invention relates to surveillance systems and more particularly to coherent frequency doppler effect radar systems for detecting minimum motion of a transponder within a detection zone limited in extent by radar signal propagation times.
2. Discussion of the Prior Art
Theft from business establishments has long been a serious and growing problem. This has been particularly true for retail establishments such as clothing stores where the trend has been to make merchandise readily accessible and to encourage self-service by the customers. While encouraging purchases and reducing labor costs, such merchandising techniques have had the undesirable effect of reducing the barriers to theft.
A known technique for discouraging theft of merchandise is to place on each merchandise item a transponder tag which receives and reradiates radio frequency electromagnetic signals. Detector systems are placed at each exit to sound an alarm whenever an active transponder tag approaches the exit. An alarm thus sounds whenever an attempt is made to carry stolen merchandise through an exit. The alarm does not sound for lawful customers because when a customer pays for selected merchandise the sales clerk uses a special tool or key to either render the transponder inactive or to remove the transponder for reuse on another item of merchandise.
The transponder can be as simple as a flat metal strip in the shape of an open loop bridged by a nonlinear circuit device such as a diode. The nonlinear circuit device causes the transponder to modulate incident waves and reradiate the sums and harmonics of the incident waves. Such a device can be enclosed within a plastic housing which is affixed to merchandise items and removed only with a special tool.
Such surveillance systems have proven highly effective in reducing theft. Experience has shown, however, that the deterrent effect of such systems derives more from the knowledge among shoppers that such a system is in place than from the ability of such a system to reliably detect theft. In fact, many of the systems presently in use are more likely to indicate a false alarm than the presence of a pilferer and store personnel often reduce the sensitivity of the system in an attempt to eliminate the false alarms. The result is that the system often fails to respond to the presence of a transponder tag when it should.
Moreover, because most surveillance systems operate on a proximity principle which causes a detector to indicate an alarm when a transponder comes close enough to reradiate a signal having a strength greater than a threshold value, care must be taken to keep merchandise a considerable distance from the store exits. Not only does this waste valuable floor space near the store exits, but an alarm will be sounded any time a customer carries a tagged item near a store exit even if there is no attempt to steal the item. Such false alarms become an embarrassment to both customers and the store proprietor and can rapidly destroy customer goodwill.
Many examples of surveillance systems can be found in the prior art. Thus, U.S. Pat. No. 4,281,321 (Narlow et al) discloses a surveillance system in which the sum of a high frequency carrier signal and a lower frequency modulating signal produced by a floor mat is detected.
U.S. Pat. No. 4,274,089 (Giles) discloses a surveillance system in which harmonics of one signal are detected instead of the sum of two signals.
U.S. Pat. No. 4,303,910 (McCann) teaches an arrangement in which a transponder tag can be excited by an incident signal at a first frequency to cause resonance and reradiation of a return signal at a second frequency. False alarms are reduced by requiring the transponder to respond simultaneously to two different incident signals radiated by different antennas on opposite sides of a surveillance zone.
U.S. Pat. No. 4,212,002 and U.S. Pat. No. 4,206,453, both to Williamson, disclose a surveillance system in which a large area is radiated with a signal of a first frequency and smaller control areas within the large area are radiated with signals of second and third frequencies which define boundaries of the smaller control zones. Detection of transponder signals reflecting all of the transmitted signals indicates the presence of the transponder within the control zone.
U.S. Pat. No. 4,117,466 (Lichtblau) teaches an arrangement in which false alarms are reduced by detecting noise interference from an interfering transmitter and inhibiting the generation of an alarm during the presence of such noise interference.
U.S. Pat. No. 3,863,244 (Lichtblau) discloses a surveillance system using an incident transmitter which sweeps through a range of frequencies and transponders which are excited by more than one frequency. An alarm is indicated only when a transponder is excited at each of its different frequencies.
U.S. Pat. No. 3,493,955 (Minasy) teaches another arrangement utilizing a transponder which resonates at a second frequency when excited by incident signals transmitted at a first frequency. A detector circuit responds to signals at the second frequency.
Skolnik, Merrill I., Introduction to Radar Systems, McGraw-Hill Book Company, 2nd Ed. 1980, discloses at pages 68-98 a doppler shift CW radar system using a quadrature phase doppler detector to detect both the magnitude and polarity of motion induced doppler frequency shifts in a received radar signal. This treatise also describes, at pages 117-119, a range-gating technique used in connection with doppler filters.
Last, U.S. Pat. No. 4,302,846 (Stephen et al) discloses a particular transponder arrangement including a nonlinear coupling element.